Allergies bug me, but call me a wimp I stick to the lessons taught in my Open Water class 36 years ago and still taught today. Don't take decongestants and dive. (That includes Afrin and Sudafed.) There have also been some humorous (because they worked out well) stories here about reverse squeeze when medication wore off. I think one involved a 5 gallon bucket being passed down to a diver on a Safety stop, along with Afrin. The bucket made a little diving bell around the guy's head so he could use the Afrin.
Instead, here's what works well for me:
1. I drink lots of water. It's worse if I'm dehydrated at all.
2. I will take Sudafed AFTER a day of diving, to start with clear ear canals over night. And repeat step 1.
3. If it's bad, I'll take Benadryl or another antihistamine the night before diving. This assumes I'm sure I don't have a cold: Antihistamines aren't a great response to a cold, as I understand, and can make the issue worse by thickening mucus.
4. I'll take ibuprofen if there's any residual pain. (Sometimes comes with squeeze due to above-water air pressure changes and clogged canals. I think this reduces inflammation in the ear canal and then, if I go back to step 1, the mucus drains away more easily.)
5. Don't forget step 1.
All that said, ask an MD/DO about this, not ScubaBoard.